"While prior animal studies found fish oil increased circulating adiponectin, whether similar effects apply in humans is not established," said the study's lead author, Jason Wu, of the Harvard School of Public Health.

"By reviewing evidence from existing randomized clinical trials, we found that fish oil supplementation caused modest increases in adiponectin in the blood of humans," Wu said.

In total, 682 subjects were treated with fish oil, and 641 were given placebos - most commonly olive and sunflower oils.

In those taking fish oil, adiponectin levels increase by 0.37 microgrammes per millilitre.

The results also found the effect of fish oil on adiponectin differed substantially across the trials, suggesting that fish oil supplementation may have stronger influence on adiponectin in some populations and weaker effects in others.

This is the first study to pool data from previous trials to suggest that fish oil consumption increases adiponectin in humans.

"Although higher levels of adiponectin in the bloodstream have been linked to lower risk of diabetes and coronary heart disease, whether fish oil influences glucose metabolism and development of type 2 diabetes remains unclear," said Wu.

"However, results from our study suggest that higher intake of fish oil may moderately increase blood level of adiponectin, and these results support potential benefits of fish oil consumption on glucose control and fat cell metabolism," Wu added.